In recent years, wholly aromatic polyamides or wholly aromatic polyesters have been receiving widespread attention as thermally stable polymers having superior properties, and accordingly, the importance of isophthaloyl dichloride or terephthaloyl dichloride as a raw material for these polyamides or polyesters has remarkably increased. The isophthaloyl dichloride or terephthaloyl dichloride used for this purpose, however, is required to have high purity. Various methods have been suggested for producing the isophthaloyl dichloride or terephthaloyl dichloride, and among them, a method for producing isophthaloyl dichloride or terephthaloyl dichloride which comprises chlorinating m-xylene or p-xylene or a compound resulting from the partial chlorine-substitution of its methyl groups to form .alpha.,.alpha.,.alpha.,.alpha.',.alpha.',.alpha.'-hexachloroxylene, and reacting the resulting hexachloro-xylene with isophthalic acid or terephthalic acid is especially advantageous from an economic standpoint. The reactions in this method are schematically shown by the following reaction equations (I) and (II). ##STR1##
We have extensively studied the method of producing aromatic dicarboxylic acid chlorides in accordance with equations (I) and (II), and found that the quality of the hexachloroxylene exerts a great influence on the quality of the aromatic dicarboxylic acid chlorides produced by the above method, and therefore, hexachloroxylene of high purity is especially desired for the production of high quality aromatic dicarboxylic acid chlorides as raw materials for thermally stable polymers.
Hexachloroxylene is prepared by a side chainchlorination reaction of xylene or a compound resulting from the partial chlorine-substitution of its methyl groups. Known methods for its production include the chlorination of xylene or a compound resulting from the partial chlorine-substitution of its methyl groups in the liquid phase by irradiation of light, or in the presence of a peroxide such as benzoyl peroxide. However, according to these methods, impurities such as a compound resulting from the direct substitution of chlorine at the benzene nucleus of xylene, or tarry materials are formed as by-products, or chlorinating decomposition or coloration frequently incidental to chlorination occurs. Consequently, the yield of the product is reduced, and long periods of time are required for the reaction, with the result that the quality or purity of the desired product is adversely affected. When hexachloroxylene containing such impurities is used as a raw material, it is difficult to prepare aromatic dicarboxylic acid chlorides having high quality feasible for use as a raw material for thermally stable polymers.
It is known to use carbon tetrachloride as a solvent, or add an alkylene polyamine, benzamide, acetamide, an aryl phosphate or sorbitol, so as to inhibit the formation of undesirable impurities. These methods contribute to some extent to the improvement of the quality of hexachloroxylene, but to no satisfactory degree. U.S. Pat. No. 2,810,688, on the other hand, discloses a process for inhibiting undesirable side-reactions which comprises continuously contacting xylene or a compound resulting from the partial chlorine-substitution of its methyl groups with a chlorine-containing gaseous stream in the liquid phase under the irradiation of light in a multiple of stages to perchlorinate the side-chain methyl groups by passing the xylene and the chlorinecontaining gaseous stream countercurrent to each other. This method, however, is still unable to afford hexachloroxylene of satisfactory purity.
We previously found that the undesirable side-reactions take place mainly in the initial stage of the reaction, and that the use of the same hexachloroxylene as the final product as a solvent can effectively inhibit the undesirable side-reactions, and based on this finding, proposed a process for producing hexachloroxylene which comprises reacting xylene or a compound resulting from the partial chlorine-substitution of its methyl groups with chlorine under the irradiation of light to perchlorinate its side-chain methyl groups wherein .alpha.,.alpha.,.alpha.,.alpha.',.alpha.', .alpha.'-hexachloroxylene as a final product is caused to be present in the reaction system from the initial stage of the reaction so as to inhibit side reactions. This process is covered by our copending U.S. patent application Ser. No. 504,989. This method makes it possible to obtain hexachloroxylene having markedly improved purity. We have, however, found that the purity of the hexachloroxylene obtained by this method is still not entirely satisfactory for use as a material for producing aromatic dicarboxylic acid chlorides which are raw materials for producing thermally stable polymers, and requires further improvement.
A customary purifying method such as distillation or recrystallization could be employed so as to improve the purity of the hexachloroxylene, but cannot solve the problem of obtaining hexachloroxylene having such a very high purity. When it is desired to purify hexachloroxylene by distillation, it is difficult to separate impurities from hexachloroxylene obtained by the conventional method because these impurities have boiling points close to the boiling point of the hexachloroxylene. Purification of hexachloroxylene by recrystallization, on the other hand, connot afford hexachloroxylene of the desired purity at low cost; namely, since hexachloroxylene is readily soluble in organic solvents (especially in the case of .alpha.,.alpha.,.alpha.,.alpha.',.alpha.', .alpha.'-hexachloro-m-xylene, its low melting point jointly becomes a problem), a large quantity of the desired final product is lost. The recrystallization method is therefore less advantageous than the distillation method.
It is an object of this invention to provide a process for producing hexachloroxylene of high purity suitable for use as a raw material for the preparation of aromatic dicarboxylic acid chlorides of high quality.
Another object of this invention is to provide a process for producing aromatic dicarboxylic acid chlorides having such a high quality as is required of a raw material for thermally stable polymers using such a high purity hexachloroxylene as a starting material.